note: do iisreset after "install" if your data access service is IIS hosted.

One more thing: If you run out of ideas with ORA-12154: TNS:could not resolve the connect identifier specifie, try re-formatting your failing record in tnsnames.ora, if you have all paranthesis etc, ok; Then know, spaces and tabs do matter!!!! And oracle engineers who wrote it, they really suck!! [Update, 16-Feb-2009] Today, the thing that helped was a restart, plsql developer was ok to connect immediately after install, but odp.net only start working after the restart.